1. Field of the Invention
The present invention relates to a thermistor having good thermal response and good heat resistance and its preparation.
2. Description of the Related Art
A thermistor is an electronic device which utilizes the change of resistance when the temperature changes, and is widely used as a temperature sensor and a compensator for an electronic circuit. The most generally used thermistor comprises a metal oxide and is used in the temperature range of 0.degree. C. to 350.degree. C. To satisfy the requirement for the thermistor which can be used at a higher temperature, the thermistor comprising SiC or B.sub.4 C which can be used in the temperature range of 0.degree. C. to 500.degree. C. has been developed. As the thermistor which can be used at a further higher temperature, the thermistor comprising diamond which is chemically stable at a high temperature and can be used in the temperature range of 0.degree. C. to 800.degree. C. has been developed. Since diamond has a thermal conductivity of 20 W/cm.multidot.K which is the largest among all substances and a small specific heat of 0.50 J/g.multidot.K, the thermistor comprising diamond is expected to have a high thermal response speed. The diamond thermistor initially comprised single crystal diamond. Although this thermistor has a high thermal response speed, it is not widely used due to difficult control of the resistance and bad processability. Since a method of forming a diamond film by a vapor phase deposition was recently established, the diamond film grown on a substrate is used in the thermistor. Since the resistance of the diamond film can be easily controlled by doping an impurity during the vapor phase deposition of the diamond film and the processability of the film is better than that of the single crystal diamond, the thermistor which utilizes diamond formed by the vapor phase deposition has been developed as the thermistor which can be used in a wide temperature range (Japanese Patent Kokai Publication No. 184304/1988).
However, in the conventional diamond film thermistor, since a volume of a substrate is usually hundred to thousand times larger than that of the diamond film, thermal response in the substrate having the low thermal conductivity dominates that in the diamond film. The conventional thermistor has a problem that the property of the diamond is not effectively utilized. The thermistor in which natural single crystal diamond or single crystal diamond synthesized at an ultra high pressure is used as the substrate and in which the diamond film is epitaxially grown has high thermal response speed, but the single crystal diamond as the substrate is not economical.